Commonmarker installed but not loaded

I am running a local instance of Gollum on my desktop machine. After a recent update from some ancient 5.x version to the latest v6.1.0 version I noticed that rendering using commonmarker does not seem to work.

1. Starting with a clean environment, doing bundle install, and then calling bundle list does not list commonmarker as in installed gem. I believe this is expected.
2. Executing gem install commonmarker in the local repository checkout installs the gem successfully. gem info commonmarker reports it as a local gem (version 2.5.0), but bundle info commonmarker says Could not find gem 'commonmarker'. After starting gollum (bundle exec bin/gollum ...), commonmarker does not seem to be used. In particular, custom options specified in a config file (per documentation here) seem to be entirely ignored.

I modified the main bin/gollum source file by adding these two lines after the initial require section (per ChatGPT suggestion):

puts "commonmarker gem installed: #{Gem::Specification.find_all_by_name('commonmarker').any?}"
puts "CommonMarker module loaded: #{defined?(CommonMarker) ? 'true' : 'false'}"

Both lines return false. At this point I have no further ideas what might be wrong and how to debug the problem. My private notes tell me that these steps - in particular gem install commonmarker - used to work in the past, but they don't seem to work now.

I am running Debian Trixie with

• ruby version 3.3.8
• gem version 3.3.7
• bundle version 2.6.7
• gollum version 6.1.0

Let me know if any additional information is needed.